1. Field of Invention
The invention relates to a method and apparatus for AC/DC conversion with continuous sinusoidal input currents, and a controlled isolated output voltage that shows one or more of the advantages of substantially reduced electromagnetic interference (EMI) noise and substantially reduced total harmonic distortion (THD). This power conversion system may, for example, incorporate the use of three-phase delta-connected inputs to further improve performance.
2. Description of Prior Art
In many AC/DC power conversion systems (e.g., telecommunications power supplies) it is necessary either to reduce significantly or avoid total harmonic distortion (THD) of the input currents to achieve a power factor close to one. Thus today, instead of diode-rectifiers, with the need for heavy and expensive passive input filters, pulse-width modulated (PWM) rectifiers with active power factor correction (PFC) are often employed. Typically, boost-type rectifiers are used to ensure continuous input currents, resulting in an output voltage that is higher than the maximum line-to-line voltage at the AC-side. Because many applications require a small, controlled and isolated DC-output voltage, the power conversion system typically includes a second stage that is a DC/DC converter, in which the DC-link voltage is identical to the (relatively high) output-voltage of the AC/DC converter at the input side.
Emission of electromagnetic interference (EMI) due to common-mode disturbances, however, remains one of the problems of pulse-width modulated (PWM) rectifier systems. With boost-type pulse-width modulation the power switches of the rectifier system are switched at a significantly higher frequency than the mains frequency. The DC-(link) output voltage is lying across the input inductance of the according phase whenever the power switch is in the ON state. This makes the input current rise or fall, and the input current can thereby be effectively controlled. However, concurrently the output voltage alternates between zero and maximum voltage at the switching frequency. Due to the high switching frequency and the high DC-(link) output voltage of the boost-type rectifier, the output side (which is the DC-link voltage of a two-stage system) resultantly has fault currents through parasitic stray capacitors that result in the emission of EMI-noise. The same is true for the DC/DC-stage that also employs pulse width modulation.
In prior art methods for AC/DC power conversion this problem of EMI-noise emission is solved by the use of bulky and expensive EMI-filters and shielding. An alternative method is to reduce the switching frequency. Reducing the switching frequency may reduce the EMI-noise, but would increase the THD, thus necessitating bulky and expensive input current filters. Other state of the art methods to reduce EMI in power conversion systems include complicated and expensive digital control systems (including so-called xe2x80x9csoft-switchingxe2x80x9d techniques), and the use of special xe2x80x9czero-power transitionxe2x80x9d switches.
Today, state-of-the-art power factor correction (PFC) power conversion systems consist of two stages, in which the AC/DC input- and/or the DC/DC output-stage present blocking voltage stress of the power switches that is equal to the high DC-link voltage. In these systems, even one power switch showing blocking voltage stress equal to the DC-link voltage results in high EMI-noise emission of the whole power conversion system.
The invention herein provides an effective apparatus and method for reducing the expense and bulk of filtering and shielding necessitated by the EMI-noise of an AC/DC power conversion system, without increasing THD. This is accomplished by combining an AC/DC converter, which, due to its power circuit configuration, presents 50% of the DC-link voltage as blocking voltage, with a DC/DC converter, which due to its power circuit configuration, presents 50% of the DC-link voltage to each of its power switches.
The present invention includes a method of reducing electromagnetic interference (EMI) and/or total harmonic distortion (THD) during multiple stage AC to DC power conversion, which may include the step of converting AC to high voltage DC by means of a three-phase/switch/level boost type pulse-width modulation rectifier as well as the step of converting higher voltage DC to lower voltage DC by means of a system of partial DC to DC converters connected in series at their primary sides, and connected in parallel at their secondary sides. Additionally, one may add the step of filtering by means of common mode filters between converting AC to high voltage DC and converting the higher voltage DC to lower voltage DC to the method of the present invention. A further step one may add is filtering by means of common mode filters after converting higher voltage DC to lower voltage DC. Another step one may add is to repeat the step of converting higher voltage DC to lower voltage DC. Additionally, one may add the step of filtering by means of common mode filters between the repeated DC to DC conversion steps.
An other object of the present invention is to provide an AC to DC power conversion system which may include a first stage of a three-phase/switch/level boost type pulse width modulation rectifier that converts AC to high voltage DC, having outputs, and at the outputs, a second stage of a system of DC to DC converters connected to each other in series at their primary sides and connected to each other in parallel at their secondary sides. This DC to DC conversion may be accomplished by using half-bridge or full-bridge pulse-width modulation type DC to DC converters. In addition one may add a pair of common mode filters at the positive and negative outputs of the first stage, and also may add a pair of series connected capacitors connecting the positive input of the second stage to the neutral input of the second stage, and the neutral input of the second stage to the negative input of the second stage. In the alternative, one may simply add common filters to one or more of the outputs of the first stage.
A method of reducing total harmonic distortion during multiple stage AC to DC power conversion, which may include the step of converting AC to high voltage DC by means of a three-phase/switch/level boost type pulse-width modulation rectifier as well as the step of converting higher voltage DC to lower voltage DC by means of a system of partial DC to DC converters connected in series at their primary sides, and connected in parallel at their secondary sides. Additionally, one may add the step of inserting common mode filters between converting AC to high voltage DC and converting the higher voltage DC to lower voltage DC. A further step one may add is inserting common mode filters after converting higher voltage DC to lower voltage DC. Another step one may add is to repeat the step of converting higher voltage DC to lower voltage DC. A final step one may add is inserting common mode filters between the repeated DC to DC conversion steps.
The present invention in its several embodiments produces superior performance over state-of-the-art systems by minimizing cost, size, and complexity while reducing THD and EMI. The invention utilizes circuit configurations rather than complex control schemes to keep EMI and THD low, efficiently and simultaneously.
It is understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the inventions claimed. The accompanying drawings illustrate several embodiments of the invention and together with the description serve to explain the principles of the invention.